Method of making pulp



United States Patent 2,987,434 METHOD OF MAKING PULP Maija L. Murray,Dayton, Ohio, and Kyle Ward, In, Appleton, Wis, assignors to TheInstitute of Paper Chemistry, Appleton, Wis., a corporation of WisconsinNo Drawing. Filed Sept. 9, 1958, Ser. No. 759,855 9 Claims. (Cl.162-157) The present invention relates generally to a method of makingimproved pulp for the manufacture of paper, and more particularly, itrelates to a method of making improved cellulosic pulp, by chemicallytreating said pulp, for the manufacture of paper having increasedtensile strength and fold endurance.

After pulp has been manufactured in any one of the several methods whichare known to the paper art, it is generally further processed to developwetness or reduce freeness; that is, the pulp is processed to induceincreased hydration of the cellulose fibers of the pulp. Freeness orwetnms is measured by the rate at which water drains from a stocksuspension through a wire mesh screen.

Low degrees of freeness are generally induced by beating the pulp untilit has a soft, slippery, slimy feel and a significantly greater waterretention than unbeaten pulp, though it will be understood that beatingmay be interrupted to provide pulp having intermediate properties.Beating is a well-known step in papermaking and involves the mechanicaltreatment of paper-making materials suspended in water. Beating isusually acco'mplished in an apparatus known as a beater. There is agreat deal of variation in beater operations, due to the particular typeof equipment that may be used, as well as to the type of pulp that isdesired. Because of these considerations, the beating time may vary froma short time to a period of as much as eight hours or more.

Longer periods of beating result in increased water retention orhydration of the pulp, but tend to weaken the strength of the resultingpaper because of damage to the cellulose fibers in the pulp due tobeater action. Accordingly, the beating operation is desirably limited.In papermaking processes, it is, therefore, often necessary to reducethe beating time below that desired, in order to decrease the amount ofdamage to the -pulp fibers to a reasonable level. However, suchreductions in beating time result in reduced hydration of the pulp.

It would be highly desirable to be able to increase the degree ofhydration of the pulp without increasing the amount of beating, and evenmore desirable to bydrate the pulp to a high degree with a minimumamount of beating. It would also be highly desirable to carry out thetreatment of the pulp relatively rapidly without having to use hightreatment temperatures and/or other expensive or inconvenient operatingconditions.

In the operation of the beater, blades are usually moved in closeproximity to a cooperating bed plate, and consequently, heaters consumesubstantial power. Of course, a reduction in this power requirement inthe manufacture of paper is desired, provided however, that pulp havingthe same or increased hydration is produced as is provided with higherpower utilization.

There has now been discovered an improved method of treating cellulosicpulp in situ so as to enhance the papermaking characteristics of thepulp While reducing 2,987,434 Patented June 6, 1961 the amount ofbeating necessary to provide a desired de gree of hydration of the pulp.Furthermore, the method may be carried out at ambient temperature, ifdesired. Such method involves subjecting the pulp, under substantiallyanhydrous conditions, before beating to treatment with a selectedreagent so as to bring about a low degree of substitution of hydrophilicgroups for hydroxyl-cdntaining radicals in the cellulose of the pulp.The method provides a pulp which is capable of making sheets ofincreased tensile strength and fold endurance.

Accordingly, the main object of this invention, therefore, is to providea method of making improved papermaking pulp, by increasing thesorptability of the pulp without substantially damaging the fibers inthe pulp. It is also an object of the present invention to provide amethod whereby improved paper products can be manufactured. It is afurther object of the present invention to provide -a method fortreating cellulosic pulp in situ to improve its papermakingcharacteristics, which method is readily carried out at a relatively lowtemperature and at a relatively rapid rate.

As will become more clearly described hereinafter, the method of thepresent invention involves treating cellulosic-containing pulp withsubstantially anhydrous reagent comprising a selected acid in dimethylsulfoxide so as to introduce into the cellulose of the pulp, to a lowdegree, hydrophilic groups comprising short chain organic radicals. Thetreatment provides the pulp with improved papermaking properties,including low freeness. Moreover, in the beater operation, the time andpower required to achieve the desired degree of freeness in the pulp areconsiderably reduced, if the pulp has been treated with the reagent inaccordance with the present invention before beating.

The pulp may be any papermaking materials which contain cellulose, forexample, cotton linter pulp, rag stock, Douglas fir pulp, or the like ormixtures thereof, with or without noncellulosic-containing constituents.The pulp, after being subjected to the described treatment in accordancewith the present invention, may, if desired, be blended in variousdegrees with other pulp prior to or subsequent to'beating.

Paper sheets produced from pulp treated in accordance with the method ofthe present invention exhibit a tensile strength which is increased asmuch as percent or more in tensile strength over that of handsheets madefrom conventionally processed pulp. In addition, the zero-span tensilestrength may be increased more than 22 percent over that forconventional sheets. Moreover, the fold endurance of such sheets hasbeen greatly increased, by the use of the method of the presentinvention, up to 400 times that of the sheets made from conventionallyprocessed pulp, further indicating that higher quality paper productsare produced in accordance with the present invention.

The observed increase in strength characteristics for the paper productsis obtained without accompanying decrease in other desirablecharacteristics of the paper products and without the production ofundesirable characteristics. Such improvement in strengthcharacteristics appears to result from the substitution of a short chainhydrophilic radical, to a low degree, for hydroxyl groups on some of thecellulose molecules of the pulp. This per- 5 mits the pulp fibers to bemore readily separated from 0.07. The degree of substitution (D.S.) isdefined as follows:

molecular equivalents of D S hydrophilic radicals substituted molecularequivalents of anhydroglucose units The anhydroglucose unit is the basicstructural unit of the cellulose,

At 0.07 to 0.10 D.S. cellulose fibers become partially soluble in water.Moreover, no further improvement in the tensile strength and foldendurance can be obtained with such high D.S. Any lower D.S. providessome improvement in the papermaking properties of the pulp but belowabout D.S. 0.009, the improvements are minimal. It is preferred that thedegree of substitution be kept Within the range of from about .03 to .05for optimal results. The degree of substitution is inverselyproportional to freeness of the pulp, which has been previouslydescribed.

The hydrophilic groups which may be substituted in the cellulosecomprise carboxyalkyl groups having carbon chains of less than fourcarbon atoms in the alkyl chain, so as to provide ethers of cellulosehaving a hydrophilic character. In general, longer chains in the alkylgroups result in reduced hydrophylic properties so that the improvedproperties of the present invention do not result. The ethers, which areformed by the substitution reaction, comprise only a small portion ofthe pulp, so as not to unduly increase the water solubility of the pulp,while still providing a large increase in the hydration of the cellulosefibers.

Consequently, the necessity for long periods of beating in thepapermaking operation to bring about hydration of the pulp at theexpense of strength characteristics in the pulp is substantiallyeliminated. The mechanism by which the desired result is accomplished isnot completely understood, but it is evident that pulp which has beentreated in accordance with the present invention has greatly increasedhydrophilic properties and may be formed into paper sheets which haveincreased tensile strength and fold endurance.

The method of the present invention involves carboxyalkylation of thecellulosic pulp in situ and under selected conditions, preferably insubstantially anhydrous environment. This latter condition preventsmercerization of the pulp during its treatment. It has been found thatmercerization of the pulp is undesirable, particularly where relativelyshort fibers, such as those of cotton linters, are utilized in the pulp,since mercerization results in a decrease in fiber length, withconsequent decrease in fold endurance when the pulp is manufactured intopaper.

The limited carboxyalkylation of the pulp in situ is accomplished byreacting the pulp with a selected acid in dimethyl sulfoxide. A speciesof the carboxy alkylation is carboxymethylation in which a carboxymethylgroup is substituted in the cellulose for a few of the free hydroxylgroups of the cellulose molecule. This type of reaction may be obtainedby reacting monochloroacetic acid with alkaline cellulose. In thismanner, some of the hydroxyl groups of the cellulose molecules react toform a glycolic acid ether having the general formula ROCH COOH, where Rrepresents the remaining large portion of the cellulose molecule ormodified cellulose molecule to which the particular OCH COOl-I group isattached.

It should be noted that the improved sorbing qualities of the cellulosicpulp obtained by in situ carboxyalkylation of the pulp are not achievedby the addition of commercially available substances such ascarboxymethyl cellulose (CMC) to the pulp before or during the beateroperation for the following reasons: (1) if CMC is merely added to thepulp, it is diflicult to control the degree of retention of CMC in thepulp because of its solubility in water; (2) the CMC which is sorbed onthe pulp may be removed by washing; (3) the hydrophilic groups providedby the CMC do not appear to be very uniformly distributed in the pulp;(4) it has been found that in certain pulp beating operations theimprovement in strength characteristics brought about by the addition ofthe CMC undergoes substantial decreases while during the same beatingoperations the strength characteristics of the partially substitutedcellulose fibers treated in accordance with the present inventionimprove.

Now referring more particularly to the method of the present invention,cellulose-containing pulp is obtained in the usual Way from variouscellulosic sources. The pulp is then dried in any suitable manner, asfor example, air drying and/or drying by solvent exchange technique todrastically reduce the Water content thereof. The dried pulp is then,before beating, subjected to in situ carboxyalkylation to a low degreeby soaking it in at least substantially anhydrous reagent comprising atleast substantially anhydrous liquid dimethyl sulfoxide and a small buteffective amount of an acid selected from the group consisting ofmonochloroacetic, monobromoacetic, monoiodoacetic, monchlorobutyric, andmonochloropropionic acids and mixtures thereof. The particular acid isselected from the group on the basis of the particular type ofcarboxyalkylation it is desired to effect. However, it is generallyprefered to carry out in situ carboxymethylation, utilizingmonochloroacetic acid.

Dimethyl sulfoxide has the formula C H OS or CHgSOCHa and is normally anoil or thick syrup with a melting point of 6 C. and a decompositionpoint of about 100 C. It is commercially available. It has been foundthat the enhanced results obtained by the method of the presentinvention are, to a considerable extent, due to the use of the dimethylsulfoxide as solvent for the carboxyalkylation. It is not known in Whatmanner the dimethyl sulfoxide operates to bring about such improvedresults. However, the improved results afforded by the use of thedimethyl sulfoxide in place of other anhydrous or substantiallyanhydrous organic solvents is demonstrable. Thus far, solvents exertingthe same eflect on the pulp as dimethyl sulfoxide have not been found.

The acids above-listed are also commercially available and are normallyliquid at room temperature. The selected acid or acids may be utilizedfor the carboxyalkylation in any suitable concentration in the dimethylsulfoxide as, for example, 2 to 4 percent by Weight, which is sufiicientto bring about the desired low degree of substitution in the pulp,within the range of from about 0.009 to 0.08, preferably .03 to .05, andwithin a reasonable period of time. The reagent is preferably present ina volume at least as great as the pulp which is soaked therein, in orderto facilitate suspending the pulp in the reagent and to assureuniformity of reaction throughout the pulp. Preferably, thereagent-to-pulp ratio is at least about 2: 1, on a volume basis.

The soaking or immersion step can be carried out at any desiredtemperature above the melting point of the dimethyl sulfoxide and acidand below the boiling point or decomposition point thereof. The soakingstep can be conveniently carried out at room temperature (65-70 F.) orbelow. It is preferred to carry out the soaking steps at ambienttemperature, usually Within the range from about 55 to F., so that heatneed not be added to the system. The carboxyalkylation reaction issufficiently rapid at low temperatures utilizing the reagent of thepresent invention, so that temperatures within the above-described rangecan be readily utilized.

The soaking step is carried out until a predetermined period of time haspassed which, depending on the type of pulp, concentration and type ofacid in the reagent, temperature and other conditions, will assure thatthe desired low degree of carboxyalkylation, that is, substitution ofthe hydrophilic groups in the cellulose, has occurred. For example,soaking cotton linter pulp in 2 parts by volume of dimethyl sulfoxidecontaining 4 percent by weight of monochloroacetic acid 1 hour at roomtemperature is usually sufficient to bring about the desired low degreeof substitution of from about 0.009 to 0.07.

After the soaking step, the pulp can be processed in any suitable mannerto the final product. In this connection, it is prefered that the excessreagent be removed, as by filtering or the like, from contact with thepulp, and that whatever residual acid is present in the pulp be thenneutralized with a suitable base, such as sodium hydroxide, potassiumhydroxide or the like.

Since it is desired to avoid mercerization of the pulp, the alkalineneutralization treatment of the pulp with the base should be carried outunder at least substantially anhydrous conditions. For example, the basecan be placed in a non-aqueous solvent, such as dimethyl sulfoxide ormethanol, or a mixture thereof, or other suitable organic solvent can beemployed. It is preferred to utilize dimethyl sulfoxide as the medium,with or without another organic solvent. Neutralization can be carriedout at any suitable temperature, for example, ambient temperature. Thepulp can then be separated from the base and non-aqueous medium, and canbe subjected to treatment at any desired temperature with an aqueoussolution of a basic water-soluble salt, for example, sodium bicarbonate,to convert the compound formed from cellulose by the carboxyalkylationreaction into the corresponding salt. After such treatment, the

6 pulp is then ready for conversion into paper having greatly improvedtensile strength and fold endurance over paper made from conventionallyprocessed pulp. The following example more particularly sets forthcertain features of the present invention.

EXAMPLE Cotton linters pulp, was solvent exchange dried in theconventional manner utilizing a mixture of water, methanol and benzene.The pulp was then air dried to remove the residual benzene therefrom,after which the pulp was separated into portions A, B, C and D of equalweight. Portion A was the control and was left untreated prior tobeating and conversion to paper. Portions B, C and D were each soaked intwo parts by weight of anhydrous reagent to one part by weight of pulp.-In the case of portion B, the reagent comprised dimethyl sulfoxide,with 2 percent, by Weight of dimethyl sulfoxide, of anhydrousmonochloroacetic acid added, while in the case of portions C and D thereagent comprised dimethyl pulp is ready for conventional beatingoperations to hydrate the pulp to the desired degree and prepare it forconversion into paper products.

It has been found that the fold endurance and tensile strength of papermade from the treated pulp can be further enhanced, subsequent to thesoaking treatment, when the alkaline neutralization step is carried oututilizing an excess (above that required for neutralization) of base,preferably sodium hydroxide. The excess base should be present in anamount sufficient to provide an excess base concentration of at leastabout 0.3 percent, by weight of the total amount of the base, preferablybetween about 0.3 percent and about'l.0 percent, to obtain the enhancedresults. Furthermore, higher percentages Within the range set forth havebeen found to be preferable.

Subsequent to such alkaline neutralization, the base and non-aqueousmedium may be separated from the pulp, as by filtering or the like. Theresidual excess base in the pulp may then be neutralized by contactingthe pulp with acid, as by immersing it in an aqueous solution containinga small amount of acetic acid or other suitable acid. Thereafter, thepulp may then be washed with water to remove the formed salts andresidual acid. The pulp is not soluble in alkaline solution at low orhigh temperature. Treatment of the pulp with a basic salt solution, forexample sodium bicarbonate, to form the corresponding salt of thecellulose derivative formed by carboxyalkylation can then be carriedout, as previously described.

Refining subsequent to the described pulp treating steps is carried outto beat the pulp to a given freeness. Such beating can be completed inless time than when untreated pulp is beaten, and furthermore, providesthe treated pulp with enhanced physical characteristics. 7 Thesulfoxide, with 4 percent, by weight of dimethyl sulfoxide, of anhydrousmonochloroacetic acid added.

The soaking treatment was carried out in the absence of agitation andover a period of one hour at ambient temperature (about 70 F.), afterwhich the reagent was separated from each portion of pulp by filteringand pressing the pulp portion.

Portions B, C and D were then each suspended in two parts of a mixtureof anhydrous dimethyl sulfoxide and anhydrous methanol. The dimethylsulfoxide was present in the mixture in a Volume ratio to the methanolof 7 to l. The mixture also contained sutficient sodium hydroxide toneutralize all of the acid remaining in the pulp and to provide anexcess sodium hydroxide concentration. In the case of portions B and C,this excess concentration of sodium hydroxide was about 0.3 percent, byweight of the pulp on a bone dry basis. In the case of portion D, theexcess sodium hydroxide concentration was about 1.0 percent, by weightof the pulp on a bone dry basis. The neutralization was carried out atabout 86 F. for a period of between about one and about two hours, thesuspension being occasionally stirred to aid the neutralization.Mercerization of the pulp did not take place, due to the substantialabsence of water in the suspension.

Following the alkaline neutralization step, the mixture was thenfiltered from each of portions B, C and D of the pulp and each portionB, C and D was suspended in a separate portion of a dilute aqueousacetic acid solution (1 N acetic acid) at ambient temperature toneutralize the excess sodium hydroxide. After a few minutes (10minutes), the dilute acetic acid solution was filtered from each ofportions B, C and D of the pulp, which portions were then water washedseveral times to remove residual salts and acetic acid.

Portions B, C and D were then suspended in separate portions of a 6percent, by weight, aqueous sodium bicarbonate solution to convert theformed carboxymethyl cellulose in the pulp portions to the sodium saltthereof. This step was carried out at ambient temperature over a periodof about 12 hours. The sodium bicarbonate solution was then drained fromeach of portions B, C and D, which were then washed until neutral in pH.

Portions A, B, C and D were subsequently refined in a conventional Jokrorefining mill, samples thereof being taken at various intervals of timeduring the refining. Each of the samples was made into standard TAPPIhandsheets which were tested for fold endurance on the standard MIT foldtester. Conventional tensile strength tests were also run on thehandsheets, and corrected zero-span tensile strengths were determined inthe usual manner for paper handsheets. The results of the various testson the sample handsheets made from treated and untreated pulp portionsare set forth in the following table. i

Table Corrected Corrected MI'l Fold Tensile zero-span Refining TesterStrength Tensile Portions Pulp Treatment D.S Time Endurance (lb./in./10lbs. Strength (min (double folds) basis Weight) (lb./in./10 lbs. TAPPIhand- TAPPI handbasis Weight) sheets sheets TAPPI handsheets 75 4 4.124.0 (A) Untreated Linters Pulp 0.003 145 4 Av.=4 5.6 Av 5 23.0 Av.=23.3

205 4 6.8 22.2 (B) 2% monochloroacetlc acid in dimethyl 75 4 4.4 23.5

sulfoxide, over neutralization with 0.3% 0.009 120 5 Av.=6 6.0 Av =5.728.5 Av.=25.7 NaOH at room temp. 170 9 6.8 25.2 (C) 4% monochloroaceticacid in dimethyl 75 31 8.4 28.7

sulfoxide, over neutralization with 0.3% 0.026 95 57 Av.=49 9.7 Av.=9 428.8 Av.=28.6 NaOH at room temp. 105 59 10.2 28.2 (D) 4%monochloroacetic acid in dimethyl 30 10 6.2 25.4

sulioxide, over neutralization with 1.0% 0.036 75 232 Av.=436 12.0Av.=11 4 28.6 Av.=27.3 NaOH at room temp. 100 1,965 15.9 28.0

The results set forth in the above table clearly illustrate the greatlyimproved fold endurance obtained by treating cellulosic pulp with thereagent of the present invention in accordance with the method of thepresent invention, before refining the pulp and converting it to paper.In this connection, handsheets made from the treated portions B, C and Ddemonstrated average fold endurance increases of 50%, 1125% and l0,800%,respectively, over the average fold endurance of handsheets made fromuntreated portion A. Handsheets made from treated portion D pulp had amaximum fold endurance increase of 49,025 over the control handsheetsmade from portion A. It should also be noted that there was a directcorrelation between refining time and increased fold endurance when pulptreated in accordance with the present invention was utilized. Thelonger the refining time the greater the fold endurance. This was notthe case with the control pulp portion A, which exhibited no increasedfold endurance with increased refining time.

Significant increases in corrected tensile strength were also obtainedwith the method of the present invention, as Will be noted from acomparison of the values listed in the table for handsheets made fromtreated portions B, C and D, with those for handsheets prepared fromuntreated pulp portion A. In this connection, TAPPI handsheets preparedfrom treated pulp portions B, C and D exhibited average correctedtensile strength increases of about 4%, 71% and 108% over the averagecorrected tensile strength of TAPPI handsheets prepared from untreatedpulp portion A. The corrected tensile strength in each case increasedwith the length of refining time. TAPPI handsheets prepared from thetreated pulp had a maximum corrected tensile strength increase of about133% over the maximum corrected tensile strength for TAPPI handsheetsprepared from the untreated pulp.

TAPPI handsheets prepared from treated pulp portions B, C and D alsowere found to have average corrected zero-span tensile strengthincreases of about 10%, 23% and 17%, respectively, over the averagecorrected zerospan tensile strength of the TAPPI handsheets preparedfrom the untreated pulp portion A. Increased refining times, up to alimit, for the pulp portions generally resulted in increased correctedzero-span tensile strength for the TAPPI handsheets made from suchrefined pulps. However, it was found that such corrected zero-spantensile strengths passed through a maximum and that further increases inrefining time resulted in slight decreases in the corrected zerospantensile strengths for the various TAPPI handsheets tested. Accordingly,in refining pulp treated in accordance with the present invention, therefining time should be selected so as to provide the optimal overallresults with respect to tensile strength and fold endurance.

The preceding example clearly illustrates that paper products ofimproved properties, particularly greatly enhanced fold endurance andincreased tensile strength, can be prepared from conventional cellulosicpulps by treating the pulps in accordance with the method of the presentinvention. Selected acids are utilized in dimethyl sulfoxide under atleast substantially anhydrous conditions to bring about substitution ofhydrophilic groups in low degree in place of hydroxyl groups in thecellulose of the pulp before beating of the pulp. This facilitateshydration of the pulp and minimizes the beating necessary to prepare thepulp for manufacture into paper products. There is no necessity ofintroducing heat into the system in carrying out the treatment of thepulp before beating. Accordingly, the method of the present inventioncan be conveniently conducted utilizing relatively simple equipment witha minimum of expense. Various other advantages provided by the method ofthe present invention are as set forth in the foregoing. The presentinvention is a distinct improvement over conventional pulp treating andpapermaking procedures.

Various modifications in the steps of the method of the presentinvention and in the materials and apparatus for carrying out suchmethod as would be apparent to those skilled in the art are contemplatedas being within the scope of the present invention.

The various features of the present invention which are believed to benew are set forth in the following claims.

We claim:

1. The method of making improved cellulosic pulp for the manufacture ofpaper of increased fold endurance and tensile strength, said methodcomprising the steps of preparing dried cellulosic pulp, treating saidpulp in situ in an at least substantially anhydrous environment byimmersing said pulp in an at least substantially anhydrous reagentcomprising a mixture of dimethyl sulfoxide and a compound selectedfromthe group consisting of monochloroacetic, monoiodoacetic,monobromoacetic, monochloropropionic and monochlorobutyric acids andmixtures thereof, said compound being present in an amount suflicient tobring about carboxyalkylation of the cellulose of said pulp to a lowdegree of substitution, less than 0.08, said pulp being maintained insaid reagent until said low degree of substitution is obtained, andsubsequently beating said pulp.

2. The method of making improved cellulosic pulp for the manufacture ofpaper of increased fold endurance and tensile strength, said methodcomprising the steps of preparing dried cellulosic pulp, treating saidpulp in situ by immersing said pulp in an at least substantiallyanhydrous environment comprising an at least substantially anhydrousreagent comprising a mixture of dimethyl sulfoxide and a compoundselected from the group consisting of monochloroacetic, monoiodoacetic,monobromoacetic, monochloropropionic and monochlorobutyric acids andmixtures thereof, said compound being present in an amount sufficient tobring about carboxyalkylation of the cellulose of said pulp to a lowdegree of substitution, less than 0.08 and more than about 0.009, saidimmersion being carried out for a time suflicient to obtain said lowdegree of substitution in said pulp, and subsequently beating said pulp.

3. The method of making improved cellulosic pulp for the manufacture ofpaper of increased fold endurance and tensile strength, said methodcomprising the steps of preparing dried cellulosic pulp, treating saidpulp in situ by immersing said pulp in an at least substantiallyanhydrous environment comprising an at least substantially anhydrousreagent comprising a mixture of dimeth yl sulfoxide and a compoundselected from the group consisting of monochloroacetic, monoiodoacetic,monobromoacetic, monochloropropionic and monchlorobutyric acids andmixtures thereof, said reagent being present in an amount of at leastabout one part by volume per part of pulp and said compound beingpresent in a concentration of at least about 2 percent, by weight ofsaid dimethyl sulfoxide, sufiicient to bring about the substitution ofhydrophilic groups for hydroxyl groups in the cellulose of said pulp toa low degree, less than 0.08 and at least about 0.009, said pulp beingmaintained in said reagent until said low degree of substitution isobtained in said pulp, and subsequently beating said pulp.

4. The method of making improved cellulosic pulp for the manufacture ofpaper of increased fold endurance and tensile strength, said methodcomprising the steps of preparing dried cellulosic pulp, treating saidpulp in situ by immersing said pulp in an at least substantiallyanhydrous environment comprising an at least substantially anhydrousreagent comprising a mixture of dimethyl sulfoxide and a compoundselected from the group consisting of monochloroacetic, monoiodoacetic,monobromoacetic, monochloropropionic and monchlorobutyric acids andmixtures thereof, said reagent being present in an amount of about twoparts by volume per part of said pulp and said compound being present insaid reagent in a concentration of between about 2 percent and about 1percent, by weight of said dimethyl sulfoxide, sufiicient to bring aboutcarboxyalkylation to substitute hydrophilic groups for hydroxyl groupsin the cellulose of said pulp to a low degree, of at least about 0.009and not more than about 0.07, said pulp being maintained at aboutambient temperature until said low degree of substitution is obtained insaid pulp, and subsequently refining said pulp by beating.

5. The method of making improved cellulosic pulp for the manufacture ofpaper of increased fold endurance and tensile strength, said methodcomprising steps of preparing substantially dry cellulosic pulp,treating said pulp in situ by immersing said pulp in a substantiallyanhydrous environment comprising an anhydrous reagent comprising amixture of anhydrous dimethyl sulfoxide and anhydrous monochloroaceticacid, said monochloroacetic acid being present in a concentration ofabout 4 percent, by weight of said dimethyl sulfoxide, sufiicient toetfect carboxymethylation of said cellulose of said pulp to a low degreeof substitution, of between about .03 and about .05, said reagent beingpresent in a volume at least equal to that of said pulp, said immersionbeing carried out at about ambient temperature until said low degree ofsubstitution is obtained in said pulp, and subsequently refining saidpulp by heating.

6. The method of making improved cellulosic pulp for the manufacture ofpaper of increased fold endurance and tensile strength, said methodcomprising the steps of preparing dried cellulosic pulp, treating saidpulp in situ by immersing said pulp in an at least substantiallyanhydrous environment comprising an at least substantially anhydrousreagent comprising a mixture of dimethyl sulfoxide and a compoundselected from the group consisting of monochloroacetic, monoiodoacetic,monobromoacetic, monochloropropionic and monchlorobutyric acids, saidcompound being present in an amount sufficient to bring aboutcarboxyalkylation of the cellulose of said pulp to a low degree ofsubstitution, less than 0.08, said pulp being maintained in said reagentuntil said low 10 degree of substitution is obtained, substantiallyseparating said reagent and said pulp, and neutralizing said separatedpulp with a base under at least substantially anhydrous conditions, andsubsequently refining said pulp by beating.

7. The method of making improved cellulosic pulp for the manufacture ofpaper of increased fold endurance and tensile strength, said methodcomprising the steps of preparing dried cellulosic pulp, treating saidpulp in situ by immersing said pulp in an at least substantiallyanhydrous environment comprising an at least substantially anhydrousreagent comprising a mixture of dimethyl sulfoxide and a compoundselected from the group consisting of monochloroacetic, monoiodoacetic,monobromoacetic, monochloropropionic and monochlorobutyric acids andmixtures thereof, said compound being present in a concentration of atleast about 2 percent, by Weight of said dimethyl sulfoxide, and in anamount sufficinet to bring about carboxyalkylation of the cellulose ofsaid pulp to a low degree of substitution, less than 0.08 and more thanabout 0.009, said immersion being carried out for a time suflicient toobtain said low degree of substitution in said pulp, substantiallyseparating said reagent and said pulp, and neutralizing said pulp with abase under at least substantially anhydrous conditions, and subsequentlyrefining said pulp by heating.

8. The method of making improved cellulosic pulp for the manufacture ofpaper of increased fold endurance and tensile strength, said methodcomprising the steps of preparing dried cellulosic pulp, treating saidpulp in situ in an at least substantially anhydrous environment byimmersing said pulp in an at least substantially anhydrous reagentcomprising a mixture of dimethyl sulfoxide and a monochloroacetic acid,said reagent being present in an amount of at least about one part byvolume per part of pulp and said acid being present in a concentrationof between about 2 percent and about 4 percent by weight of saiddimethyl sulfoxide, the amount of said acid being suflicient to bringabout the substitution of lrydroph-ilic groups for hydroxyl groups inthe cellulose of said pulp to a low degree, less than 0.07 and more thanabout 0.009, substantially separating said reagent and said pulp,overneutralizing said pulp with a water-soluble base under at leastsubstantially anhydrous conditions, the excess amount of said base beingat least about 0.3 percent, by weight of the total amount of base,neutralizing said excess base, Water washing said pulp to remove solublesalts formed therein, and refining said pulp by beating.

9. The method of making improved cellulosic pulps for the manufacture ofpaper of increased fold endurance and tensile strength, said methodcomprising the steps of preparing dried cellulosic pul treating saidpulp in situ in a substantially anhydrous environment by immer said pulpin anhydrous reagent comprising a mixture of dimethyl sulfoxide andmonochloroacetic acid, said reagent being present in an amount of abouttwo parts by volume to one part of said pulp, said acid being present insaid reagent in a concentration of between about 2 percent and about 4percent, by weight of said dimethyl sulfoxide, suflicient to bring aboutcarboxyalkylation to substitute hydrophilic groups for hydroxyl groupsin the cellulose of said pulp to a low degree, of between about 0.03 and0.05, said pulp being maintained at about ambient temperature until saidlow degre of substitution is obtained in said pulp, substantiallyseparating said reagent and said pulp, overneutralizing said separatedpulp with anhydrous solution comprising dimethyl sulfoxide and awater-soluble base, the excess amount of said base being between about0.3 and about 1.0 percent, by Weight of the total amount of base,neutralizing said base and water washing said pulp to free said pulp ofwater-soluble salts, treating said pulp with an aqueous solution ofsodium bicarbonate to convert the carboxymethylation compounds formed inthe cellulose of said pulp to their sodium salts, and subsequentlyrefining said 2,618,632 Klug ,Nov. 18, 1952 pulp by beating. 2,636,879Branan et a1. Apr. 28, 1953 2,787,606 Coover et a1 Apr. 2, 1957 OTHERREFERENCES Walecka: An Investigation of Low Degtee of SubstitutionCarboxymethycelluloses, TAPPI, vol. 39, N0. 7,

References Cited in the file of this patent UNITED STATES PATENTS2,365,898 Morris et'al Dec. 26, 1944 2,404,722 Houtz' July 23, 1946July, 1956. 2,510,355 Waldeck June 6, 1950

1. THE METHOD OF MAKING IMPROVED CELLULOSIC PULP FOR THE MANUFACTURE OFPAPER OF INCREASED FOLD ENDURANCE AND TENSILE STRENGTH, SAID METHODCOMPRISING THE STEPS OF PREPARING DRIED CELLULOSIC PULP, TREATING SAIDPULP IN SITU IN AN AT LEAST SUBSTANTIALLY ANHYDROUS ENVIRONMENT BYIMMERSING SAID PULP IN AN AT LEAST SUBSTANTIALLY ANHYDROUS REAGENTCOMPRISING A MIXTURE OF DIMETHYL SULFOXIDE AND A COMPOUND SELECTED FROMTHE GROUP CONSISTING OF MONOCHLOROACETIC, MONOIODOACETIC,MONOBROMOACETIC, MONOCHLOROPROPIONIC AND MONOCHLOROBUTYRIC ACIDS ANDMIXTURES THEREOF, SAID COMPOUND BEING PRESENT IN AN AMOUNT SUFFICIENT TOBRING ABOUT CARBOXYALKYLATION OF THE CELLULOSE OF SAID PULP TO A LOWDEGREE OF SUBSTITUTION, LESS THAN 0.08, SAID PULP BEINGMAINTAINED INSAID REAGENT UNTIL SAID LOW DEGREE OF SUBSTITUTION IS OBTAINED, ANDSUBSEQUENTLY BEATING SAID PULP.